In view of the fact that copper has higher electromigration resistance, and thus reliability, as well as higher conductivity than aluminum wiring, copper interconnects are being widely accepted as standard interconnect technology in semiconductor device fabrication. However, for successful implementation of copper interconnect technology, proper diffusion barrier materials with high thermal stability, low resistivity, and low reactivity with copper are essential since copper has higher diffusivity than aluminum and copper easily diffuses through dielectrics, producing deep level defects which are detrimental to the performance of CMOS transistors. It is for this reason that a thin tantalum/tantalum nitride (Ta/TaN) bilayer is usually used as a diffusion barrier to encapsulate the copper lines and vias used with copper interconnect features. Currently ionized physical vapor deposition (I-PVD) is the conventional deposition technique used for the formation of these Ta/TaN layers. However, the use of PVD technologies for these layers is not expected to be applicable for technologies using nodes smaller than approximately 45 nanometers due to limited ability of the deposited films to conform to these smaller features.
Recently, as an alternative deposition technique, atomic layer deposition (ALD) has been widely studied. Due to the inherent atomic level control and self-saturation chemistry, the films formed by ALD are highly conformal and uniform. While it is possible to deposit metallic Ta films using ALD with a halide tantalum precursor, the ALD of Ta and TaN as a bilayer has been only rarely successful for two reasons. First, the conventionally available ALD of TaN using an inorganic source (such as TaCl5), reacted with ammonia (NH3) has been reported to produce a highly resistive phase of tantalum nitride, the Ta3N5 phase, which presents practical difficulties for the use of these films as barriers in conducting circuits. Second, while it is possible to deposit TaN films using a metal-organic Ta source such as, for example, TBTDET (tertbutylimidotris(diethylamido)tantalum), it is impossible to use this precursor for the deposition of a metallic Ta film by ALD since the TaN bond cannot be reduced in this chemical system.
Tantalum plasma enhanced ALD (PE-ALD) using chlorides and atomic hydrogen as the metal precursor and reagent has been developed recently. An excellent explanation of ALD and apparatus for performing ALD may be found in Plasma-Enhanced Atomic Layer Deposition of Ta and Ti For Interconnect Diffusion Barriers by S. M. Rossnagel, J. Vac. Sci. Technol. B18(4), July/August 2000, which is incorporated herein by reference in its entirety.